coverity#1437954 Dereference null return value

turn DBG_ASSERT into a real assert

since...

commit 904c97bb
Date:   Mon Jul 2 17:23:59 2018 +0200

    move SvTreeList::*Sibling to SvTreeListEntry

    since they don't depend on SvTreeList at all

Change-Id: Ib230ff2fc9ab1f68c6686c125dbf4213e77bea84
Reviewed-on: https://gerrit.libreoffice.org/57457
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

svtools/source/contnr/svimpbox.cxx

@@ -1075,7 +1075,7 @@ void SvImpLBox::DrawNet(vcl::RenderContext& rRenderContext)
             aPos1.AdjustY(nEntryHeightDIV2 );
 
             pChild = pView->FirstChild( pEntry );
-            DBG_ASSERT(pChild,"Child?");
+            assert(pChild && "Child?");
             pChild = pChild->LastSibling();
             nDistance = static_cast<sal_uInt16>(pView->GetVisiblePos(pChild) - pView->GetVisiblePos(pEntry));
             aPos2 = aPos1;